1. Field of the Invention
The present invention relates to a voltage or current generator, and more particularly, to a zero-temperature-coefficient voltage or current generator.
2. Description of the Prior Art
Generally speaking, an analog circuit often needs a reference voltage circuit not influenced by supply voltage and temperature variation for improving yield of the analog circuit, reliability, and accuracy. The reference voltage circuit is known as a “bandgap reference circuit”. The bandgap reference circuit provides a voltage level (bandgap reference voltage) for other function blocks, such as an output voltage level of a regulator, or turning-on or turning-off of a battery charger, and it is a widely used and important circuit. The bandgap reference voltage is generated by adding a proportional to absolute temperature (PTAT) voltage to a complementary to absolute temperature (CTAT) voltage. The CTAT voltage is generated through a base-emitter voltage of a forward-biased bipolar transistor, and the PTAT voltage is generated through using a voltage difference between base-emitter voltages of two bipolar transistors, wherein currents flowing through the two bipolar transistors are the same, but the base-emitter voltages of the two bipolar transistors are different. Thus, the bandgap reference circuit has a low correlation with the supply voltage and process parameters, and the bandgap reference circuit is independent of temperature. Thus, the bandgap reference circuit is widely used in the analog circuits.
Temperature may influence diodes, resistors, capacitors, and transistors to different degrees. However, design of a mixed-signal integrated circuit (IC) requires more complexity, lower voltage and higher speed on an uneven power-density chip, which increases temperature gradient of the chip. Therefore, an IC designer must consider influence of the temperature gradient on the whole chip, because the analog circuits may be very sensitive to the temperature difference, even only a few degrees Celsius. But, current zero temperature-coefficient voltage and current technologies may not consider temperature effect on resistors, so that the reference voltage still correlates with the temperature, and accuracy of the reference voltage is influenced.